A numerical analysis of the flow field surrounding a solar chimney power plant

Harris, Rhydar Lee (2004-04)

Thesis (MScIng)--University of Stellenbosch, 2004.

Thesis

ENGLISH ABSTRACT: This study investigated the flow field above a proposed solar chimney power plant, without a cross wind, using a commercial numerical solver in the form of CFX 4-4 by AEA Technology plc. The governing equations solved are for an incompressible steady state solution. Variation in density due to buoyant effects is modelled with the Boussinesq approximation, and turbulence is approximated by the k-ε model with modifications due to buoyancy. The effect of different turbulence conditions at atmospheric inflows is also investigated. Modifications to the k-ε turbulence model in the context of atmospheric turbulence are investigated and applied to the standard k-ε model. These modifications include the addition of source terms in the turbulence kinetic energy and the turbulence dissipation rate equations to allow for the production/destruction of turbulence due to buoyancy. Other modifications include an additional source term in the turbulence dissipation rate equation accounting for atmospheric stability and the specification of model constants relevant to atmospheric flows. Initial results for the flow field using the Boussinesq approximation show reasonable correlation between the current study and the study by Thiart (2002) whereby the flow field exhibits characteristics of the axially symmetric turbulent jet. One of the primary and most noticeable differences between the current study and that of Thiart (2002) is the difference in height at which air is drawn into the collector. In order to account for variation in density with height, a modification on the Boussinesq approximation, the Deep Boussinesq model, Montavon (1998), is applied to a simplified 100 m by 100 m rectangular geometry. The results obtained are compared to a similar model using the Boussinesq approximation and it is seen that the average velocities reached in the deep Boussinesq model are significantly larger than those obtained in the Boussinesq approximation.

AFRIKAANSE OPSOMMING: Hierdie studie ondersoek die vloeiveld bo 'n aangewese son-skoorsteen kragstasie, deur die gebruik van 'n kommersiële numeriese oplosser in die vorm van CFX 4-4; van AEA Technology plc. Die heersende vergelykings wat opgelos moet word, is vir 'n niesaampersbare, tyd-onafhanklike oplossing. Die wisseling in digtheid as gevolg van saamdrukbaarheids effekte, word gemodelleer met die Boussinesq benadering en turbulensie-skatting deur die k-ε model met aanpassings as gevolg van dryfkrag. Die effek van die verskillende turbulensie-toestande teen atmosferiese invloeing word ook ondersoek. Aanpassings op die k-ε turbulensie-model in die konteks van atmosferies turbulensie word ondersoek en toegepas op die standaard k-ε model. Hierdie veranderinge sluit die byvoeging van bron terme in die turbulente kinetiese energie en die turbulensieverspillings vergelykings om ruimte te laat vir die produksie/afbreking van turbulensie as gevolg van dryfkrag in. Ander aanpassings sluit in 'n bykomende bron term in die turbulensie-verspillings vergelyking wat rekenskap gee vir atmosferiese stabiliteit en die spesifikasie van model onveranderlikes met betrekking tot atmosferiese vloei. Aanvanklike resultate vir die vloeiveld met die gebruik van die Boussinesq benadering dui op geredelike ooreenstemming tussen de huidige studie en die studie van Thiart (2002), waar die vloeiveld eienskappe toon van 'n simmetriese turbulente spilpunt. Een van die mees primêre en duidelikste verskille tussen die huidige studie en diè van Thiart (2002) is die verskil in die hoogte waarteen die lug in die samesteller ingesuig word. Om rekenskep te kan gee vir die verskil in digtheid met betrekking tot hoogte is 'n aanpassing van die Boussinesq benadering, die Diep Bussinesq model, Montavon (1998), aangebring. Hierdie aanpassing word toegepas op ‘n vereenvoudigde 100 m by 100 m reghoek. Die resultate wat verkry word, word vergelyk met 'n soortgelyke model waar die algemene snelheid wat met die diep Boussinesq model bereik word merkbaar groter is as die in die Boussinesq benadering.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/16337
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